Surface charge density of maghemite nanoparticles: Role of electrostatics in the proton exchange

In the present work, we focus on the surface charge properties of well-defined maghemite (gamma-Fe2O3) nanoparticles dispersed in water. Because of the acidic properties of surface groups, the nanoparticles' surface can be positively or negatively charged depending on the pH. Consequently, there are electrostatic repulsion forces between particles that allow stabilizing the suspensions in water. Acid-base titrations of the surface groups starting from acidic, alkaline, and neutral pH are performed to access the dependence of the surface charge density with the pH. Titrations are followed by potentiometric and conductometric measurements. Simultaneous zetametry measurements by acoustophoresis are implemented. The experimental curves are analyzed using several hypotheses; in particular, we assume that the sites on the surface of the particles behave as weak diacids. The interpretation of the experimental determinations of the charge implies taking into account an electrostatic term due to the potential developed at the surface of the particles. Thanks to the complementarity of the potentio-conductometric titrations and of the zetametry measurements, it is possible to better understand the phenomena of protonation and deprotonation of the colloidal particles as a function of the medium of dispersion (acidic, alkaline, neutral pH, and nature of the counterions) and of the nature of the reagent.